Defying Gravity: When Strange Liquids Act Like Solids

Walk across water and people may start to think you can work miracles. But dashing atop the surface of certain liquids has an entirely down-to-Earth scientific explanation.

New evidence suggests that the stress of walking across oobleck, a cornstarch-and-water mixture traversed by foot in the video above, compresses it into a solid-like material that can support a person’s weight.

Oobleck is what’s known as a non-Newtonian fluid, a class of relatively common liquids that behave in unexpectedly non-liquid ways. Ketchup, which flows from a bottle yet gathers neatly in a line on your hot dog, is another example. So are paint, toothpaste, and shampoo, each of which will sometimes flow and sometimes take firmer form.

Of all the non-Newtonian fluids, however, oobleck is one of the strangest. If you dip your hand in slowly, it behaves like a normal liquid — but if you move too fast, it forms a solid mass. Teachers and fun-loving college students have long known that you can fill a tub with oobleck and run across without sinking, yet previous scientific explanations for the effect have come up short.

Many researchers thought increased viscosity might be responsible, but the force created during this viscosity increase is insufficient to keep a human body aloft. So researchers have looked for other explanations.

“When you’re running across the stuff, you’re not shearing it so much as compressing it,” said graduate physics student Scott Waitukaitis of the University of Chicago, who co-authored a paper on this subject published in Nature on July 12. “So we wanted to see how the stuff responds under impact.”

Working with his adviser, physicist Heinrich Jaeger, Waitukaitis performed several experiments to more deeply investigate oobleck’s properties. Rather than measure its flow — the typical approach of physicists working with non-Newtonian fluids — the researchers shot a solid rod at oobleck and filmed the process with a high-speed camera. An accelerometer also measured the rod’s force as it struck the surface.

The results suggested that cornstarch particles suspended in the water were being shoved together, forcing them to behave like a hard object. The liquid bunched up like snow in front of a snowplow, forming a quasi-solid column.

To get a better inside-look, Waitukaitis also observed the liquid with a dental X-ray machine, using tracer particles – basically nuts and bolts they had sitting around the lab – to highlight oobleck’s inner structure. This revealed further evidence of a columnar support structure.

Waitukaitis and Jaeger calculated that this semi-solid column provided just enough force to hold up the weight of a human, provided they moved fast enough.

“This is an important observation because it allows for a lot more force so this fluid can hold up a lot more weight,” said physicist Eric Brown from the University of California, Merced, who works on non-Newtonian fluids but was not involved with this research.

But Brown thinks that there is still more to the story. The temporary solid column might not support a human if it didn’t reach to the bottom of the tub, which is likely also holding some of the person’s weight, he said.

Waitukaitis disagrees with this interpretation, suggesting that even if a human were to run over an ocean of oobleck — where the solid column didn’t touch the bottom – they would still be supported.

But both researchers agree that studying non-Newtonian fluids is important. Beyond messiah-like party tricks, mixtures like oobleck could one day be used to save lives. Already, several laboratories are attempting to inject non-Newtonian fluids into Kevlar, essentially creating liquid body armor. It’s thought that such materials could stop bullets while remaining light and fluid enough to provide easy movement.

Videos: 1) Youtube/alesis69 2) High-speed video of a metal rod striking the surface of a mixture of cornstarch and water. Scott Waitukaitis, 2011.